1. Field of the Invention
The present invention relates to a hydraulic controller and, more particularly, to a hydraulic controller provided with a function to decrease an internal leakage of a cylinder port of a spool valve.
2. Description of the Prior Art
A hydraulic system for industrial equipment including civil engineering and construction equipment and agricultural equipment is equipped with an oil hydraulic pump and a hydraulic actuator for oil hydraulic motor, hydraulic cylinder and the like, and a control valve is further provided between the oil hydraulic pump and the hydraulic actuator.
A spool type directional control valve (hereinafter simply called "spool valve") is used generally for the aforementioned control valve, operating for line switching and throttle switching so as to run the hydraulic actuator forward and backward or stop it halfway.
Since the spool valve is framed to slide a bore of the valve body, an appropriate clearance is present between the bore and the spool. Accordingly, it is unavoidable that an internal leak may arise in an internal passage of the valve body and particularly in a cylinder port. Particularly in a double-acting actuator, such internal leak is capable of deteriorating a load carrying capacity. That is, the cylinder port is connected to the actuator circuit, and a load pressure of the actuator is enclosed in the cylinder port. Accordingly, the load pressure leaks to a tank port on low pressure side along a clearance between a neutral spool outer peripheral portion and a bore, and thus a load cannot be retained and the actuator moves bit by bit according to the lapse of time.
For countermeasure it is conceivable that a poppet be used instead of the spool valve, however, a valve construction becomes large-scaled, which inevitably leads to a high cost. Now, therefore, a spool valve provided with a pilot check valve on a passage in a valve body reaching a cylinder port is disclosed in Japanese Patent Publication No. 26598/1977.
According to the prior art, a presence of the pilot check valve is effective in minimizing an internal leak, and thus a load retention of the cylinder which is an actuator is realizable. However, the prior art comprises an internal pilot system for pushing the pilot piston on a pump pressure. Thus, the piston must be enlarged in size for fine flow control. Practically, however, the pilot piston cannot be made so big in size for space requirement in the valve body.
Consequently, after the notch of a main spool passes mostly, pressure rises to actuate the pilot piston, a check valve then opens, and thus it is unavoidable that a fairly big shock may arise. Accordingly, a problem is such that a pertinent nonleak mechanism with high flow controllability cannot be realized.
Further, in the prior art, the pilot piston and the check valve are disposed opposite each other across a cylinder passage (the pilot check valve being incorporated in the valve body). Then, when the main spool moves, a hydraulic fluid is fed near a hydraulic fluid return side passage on a counter side across a cylinder port passage on a feed side, thus pushing and moving the pilot piston. A passage construction of the valve body becomes complicate consequently.